Combined flash and vapor compression evaporator



Allg 21. 1956 E. P. WORTHEN ErAL 2,759,882

COMBINED FLASH AND VAPOR coMPREssloN EvAPoRAToR Filed July 30, 1954 2 SheetsSheet l Aug. 21, 1956 E. P. woRTHl-:N ET AL 2,759,882

COMBINED FLASH AND VAPOR COMPRESSION EVAPORATOR Filed July 30. 1954 2 SheetsSheet 2 INVENTORS ATTORNEY United States Patent COMBINED FLASH AND VAPOR COMPRESSION EVAPORATOR Eugene Porter Worthen, Braintree, and Fenner Smith lBarbour, Wollaston, Mass., assignors to Bethlehem Steel Company, a corporation of Pennsylvania Application July 30, 1954, SerialNo. 446,788

9 Claims. (Cl. luz-45) This invention relates to an improved apparatus and method for evaporating and distilling, particularly adapted for producing fresh water from salt water.

An object of the invention is to provide an vevaporator of the multi-stage flash type, in which vapor evolved in the first stage is compressed and the heat of the compressed vapor is used to impart heat to the feed-water entering the first stage by out of contact heat exchange.

Another object is to provide an evaporator of the cornbined vapor compression type and multi-'stage flash type, having characteristics not possessed by either type alone.

Another object is to provide an evaporator of the com- Vbined'vapor compression type and multi-stage ash type, inV which provision is made for recirculation of a portion of lthe water not evaporated in the first stage.

Other objects will appear from the following 4descrip- *tionand claims.

The combination of the multistage flash principle of :evaporation with the vapor compression principle yof evaporation, each in itself a highly eiiicient process-re- :sults in a distilling unit of extremely' high efiiciency, having a fresh water to oil ratioofbetter than =100 .andra freshwater to steam ratio of .better than 8. Itis :awell :known fact that the multistage flash evaporator requires :Ianxexternal heater in which the feed water\is;heated,by -'means.of steam, from the temperature atwhichtit leaves ..the.1rst stage condenser to a suitable higher temperature before it is 'flashed in the first stage. This heater is generallyknown .as the salt water-heater or-evaporator `:feed heater. lt is a further well known fact that-any .iformxofvapor compression evaporator, either of the flash or submergedttube type, requires for its efficient operationfan Ieective brine and distillate cooler to recover the h'eat in the outgoing brine and condensate Aand transfer it'tothe incoming feed. In the past, this brinecooler ihaszbeen'a constant source of trouble due toscale formavtion. lBy combining the iiash and `vapor compression distillation cycles, in the manner hereinafter described, we'have employed'the vapor compressionevaporator `as a feed heater for the multistage flash evaporator andthe filash/evaporator as a brine and distillate coolerfor the vapor compression evaporator. 1

In the accompanying drawings, Fig. l is a diagram- Vmaticdrawing of the improved evaporator, `showing a '.typicalflow; sheet and heat balance, and Fig. 2 isa drawing Lof `an improved evaporating chamber and Weir for .carrying out the invention.

The manner in which the ash and vapor compression :systems `are combined is shown in the accompanying wFig. A1in which reference numerals 1, `2, 3, 4, 5, 6 and 7 representthe several evaporating chambers of 'a multi- -stage vflash evaporator. Salt water enters the system ldensers 3a, 4a, 5a,

Ithrough `conduitt and is circulated by means ofrpump 9 y liiash chamber turn is .driven by steam from 'haust steam from the turbine "heat to the feedwater.

=is condensed in condenser 2a.

,pressed vapor salt water heater '12,

Salt water leaving condenser 2a, passes through air ejector condenser 2li and through the compressed vapor salt Water heater l2 and turbine exhaust heaterl, where its temperature `is further raised to a suitable temperature for hashing in Hash chamber 1, by out of contact heat `exchange with vapors from the vapor compressor and turbine to be described below.

Salt water ieaving the turbine exhaust heater 13 enters l where a portion of it flashes into vapor `through the orifices 14 of the llash box 115. The vapor passes through perforated horizontal plates 16, lwhere a portion of the water droplets therein is removed and `thence thro-ugh the vapor separator 17 where additional of water .is drawn off through conduit 1`8 into compressor '19 where it is compressed and thereby further heated. The compressed and heated vapor passes through conduit Ztl into the compressed vapor salt water heater I2and gives up its heat to the salt water.

The `compressor 19 is driven by turbine 21 which in a boiler (not shown). Ex-

passes through! conduit 22 into the turbine exhaust heater i3 where it givesupits The condensate is withdrawn through conduit 24 and pump 25 to the boiler.

Reverting to the flash chamber il, saltwater whichris not Vvaporized therein overllows the "Weir 26formed"by one edge of the flash box 15 and enters the chamber 27. A portion of this unvaporized salt water is withdrawn lfrom` chamber 27, as described below while the remainder 'ows over a second weir 2S, through the loop sea1i129 and into the second stage ilash chamber Z, which is mai-ntained at a lower pressure than ash chamber l-and where a further part of the salt is flashed into vapor in the same manner as in the flash chamber-1. This vapor The uniashed salt water passes through successively into flash chambers 3, 4, `5,16 Aand 7, which are maintained at successively lower pressures, and in each of which it is further-vaporized. The vapors formed in said chambers are condensed in conoa and 7a. Condensate formed in each condenser flows through conduit 30 into thenext -succeeding condenser where it mingles with the condensate formed in the latter condenser.

Condensate formed heaterows through Condensate formed in in the compressed vapor salt water conduit 31 into condenser Za.

water through conduit 32, distillate pump S3 Vand distillate cooler lil.

The part of the feedwater remaining unilashed in chamber 7a is discharged to waste through conduit 3S and salt water discharge pump 36.

An essential feature of this system is that a portion of the unevaporated feedwater discharged'from chamber .l is returned through conduit 37 and pump 38 to conduit 8 where it joins at point 39 the feedwater flowing from the condenser 2a through conduit 8 into com- By means of this recirculation of feed, the amount of which can be varied, the system can be kept in proper balance, as will be explained below.

Each of the condensers 2a, 3a, 4a, `5a anda is vented into the next succeeding condenser through vents 2b, 3b, 4b, 5b and 6b. Condenser '7a is vented through Vent 7b to air ejector tl, the discharge from which is condensed in air ejector condenser 11.

Fig. 2 discloses in greater detail than Fig. l the construction of the evaporator and particularly the means. for controlling the disposition of the effluent from the rst Stage of the evaporator.

VI11"I-`ig.`2, the lirst and second stage evaporati'ng chambers are designated by the numerals 1 and 2 and are separated by the common wall 41.

Within chamber 1, flash box 1S' is provided with sidewalls 48 which form an overow weir 26 for feedwater fed thereto.

Below the flash chamber 1 is the feedwater inlet box formed by plates 42, 43 and 44. Feedwater enters said box through feedwater inlet 45 and rises through orifices 14 in the plate 46 forming the bottom of the flash box 15 where a portion of the feedwater flashes into vapor. The vapor passes through perforations 47 in the horizontal plates 16 and through the head of water in the ash box 15, whereby a portion of the water droplets contained in the vapor is removed therefrom. From there the vapor rises through the vapor separator 17 where additional moisture is removed thereupon. The vapor thus freed of moisture is drawn off through conduit 18 leading to the vapor compressor (see Fig. l).

Spaced laterally between the wall 44 of the inlet box and the common wall 41 between the iiash chambers 1 and 2 is the wall 49 which forms a second overflow weir 28. Feedwater unevaporated in flash chamber 1 overflows weir 26 of flash box 15 and enters chamber Z7 formed by walls 49 and 44. A portion of this feedwater is withdrawn from chamber 27 through conduit 37, while the remainder overiiows Weir 28 into the loop seal 29 formed by the wall 49 and the wall S0, which is a downward extension of the Wall 41 separating flash chambers 1 and 2. The feedwater flowing through loop seal 29 is further evaporated in flash chamber 2 and in the succeeding flash chambers in the manner hereinabove described.

The amount of water withdrawn from chamber 27 and recirculated, and thus the amount of water overowing the weir 28 and passing through the remaining flash chambers, can be controlled and varied by means of reeirculating pump 38.

The system Will now be described in connection with an assumed set of conditions. The figures given below are approximate.

Forming part of Fig. 1 is a typical heat flow diagram of a combined flash and vapor compression evaporator cycle calculated for 350 p. s. i. g. steam pressure, 750 F. total temperature steam supply, and 84 F. sea water temperature conditions. All quantities are based on l# of steam supplied to the cycle. Approximately 8.2# of fresh water per pound of steam, or 115# of fresh water per pound of oil, are produced (assuming a boiler etiiciency of 88%, a turbine etiiciency of 80% and a compressor eiiciency of 85%).

i Approximately 67.5# of salt Water feed at 84 F. flows 1n succession through the distillate cooler 10 and through the tubes of the vapor condensers 7a through 2a inclusive I and air ejector condenser. The flash vapor in each of these stages is condensed on the tube of the vapor condensers and gives up its latent heat to the salt water feed which, iu turn, is heated to l53.6 F. at the outlet of the air ejector condenser. At point 39 in the cycle the feed is joined by 141.5# of salt water at 170 F. recirculated from the first stage. The combined flow, 209# at 164.7 F., passes in succession through the tubes of the compressed vapor salt water heater 12 and the turbine exhaust heater 13 Where it absorbs the heat of condensation of the compressed vapor and the turbine exhaust and increases in temperature to 190 F.

The combined iiow at 190 F. then enters the first stage which is at a pressure of 6 p. s. i. a. and a temperature of 170 F. The entering salt water yin reducing to this pressure and temperature releases 4.2# of fiash vapor which passes through a vapor separator where entrained moisture is removed and enters the suction of the vapor compressor. t is compressed to 11.6 p. s. i. a. and 200.3 F. and discharged to the compressed vapor salt water heater where it condenses and transfers its heat of condensation to the incoming salt water. The resulting 4.2# of condensate which is part of the fresh water product then flows in succession through the various stage condensers where it mixes with the condensate produced in those stages and finally emerges from the seventh stage condenser.

Of the unfiashed salt water remaining in the first stage 141.5# is recirculated as previously described and 63.3# passes successively through stages two through seven inelusive in each of which further Hash vaporization takes place. The vapor flashed in each of these stages is condcnsed in its associated stage condenser. The resulting condensate iows in succession through the remaining condensers to the seventh stage where the total condensate (8.204%) is removed by a distillate pump and becomes the fresh water product. The uniashed salt water, 5930i?, is removed by the salt water overboard pump from the seventh stage and discharged to waste.

Several methods of removing air and non-condensible gases from the system are possible. For example, each of the stage condensers as well as the two heaters may be vented to the seventh stage condenser and the gases removed from the seventh stage condenser by means of an air ejector.

From the foregoing, it can be seen that of the 67.5 lbs. of feed water at l53.6 F. delivered from the air ejector condenser, 63.3 lbs. are returned to the second stage flash chamber at F. while 4.2 lbs. are ashed to fresh water, condensed and returned at 200.3 F. To accomplish this a pound of steam is required by the turbocompressor. Had this steam been used in a conventional evatorator feed water heater (salt water heater) it would have heated the feed to approximately the required 170 F. but no additional fresh water would have been produced. Therefore, by the substitution of a vapor compression component for the conventional salt water heater approximately 4.2 lbs. of additional fresh water are obtained per pound of primary steam. Since the remaining six stages of ash evaporation produce 4 lbs. of fresh water, the total fresh water produced per pound of steam is approximately doubled to 8.2 lbs. This gives a steam to fresh water ratio of 0.122.

Recirculation of a portion of the salt water from the first stage through the compressed vapor salt water heater and the turbine exhaust condenser allows the production of approximately one-half of the total fresh water in the vapor compression portion of the cycle without exceeding a maximum salt water temperature of F. It also reduces the pressure differential against which the vapor compressor must work thereby increasing the quantity of vapor that can be compressed for a given power input. This, in turn, improves the efficiency of the cycle.

The advantage of combining the flash evaporation and vapor compression principles arises from the fact that it permits attaining an economy in seven stages with a maximum saltwater temperature of 190 F. which would require eleven or twelve stages and a maximum salt water temperature of approximately 270 F. With a straight flash evaporator cycle. Such a high salt water temperature would create a difficult scaling problem.

The temperatures and pressures in the vapor compression part of the cycle are selected on the basis of both thermal and economic considerations. In the example given above, the six stage ash component of the plant (stages 2 to 7 of Fig. l) requires a salt water feed temperature of 170 F. This determines the temperature in the flash chamber of the vapor compression part of the cycle (first stage ash chamber). The temperature and pressure to which the vapor is compressed is determined by economic considerations of efficiency as well as size and cost of the various components. As the temperature and pressure are increased, the Work of compression also increases and the overall efficiency of the unit decreases. On the other hand a low compression ratio results in a smaller output for the same amount of salt water circulated, larger heating surfaces and a generally larger f plant for the same capacity. In the cycle shown in Fig. 1,

,pressure. -the compressionratio against Whichthe' compressor must operate, which in turn Would'increasefthe powerrequired fromthe turbine whichwwouldstill further raisethe 'pres- :sure and-temperature in the turbinefexhaustr'and compressed vapor `saltwater heaters ;to the-point where the a vcompression ratioresultinigina vapor temperature rise f.of 30 Panda total,feed,llash .of-2021:..from.,l90F.

to 170 F., was selected. On the basis of one poundxof motive steamnto the `|turbine, l.the-,compressor :will pump 4.2. lbs. of.vapor-between .thelimitschosem i.e^.e.,from -6.'0

this vapor-plus the V'.heat, of condensatiojmof-,the'turbine exhaust steammusbheat the feed. to 190 F `Sufflicieut ,feed at .170 1F.` is, therefore,.recirculated tov-maintainzthe feed entering theiirststage flashchamberat a temperature vof about.l.9,0,F. It is the, use of recirculationfi-n.the-vapor `stageiflash component of ,thelplant can be compensated for, thefeed enteringthe first stageflash. chamber .can .be

`maintained-at 190 F., andthe output of thecyelecan -be-held ,substantially constant.

Without recirculation theflash andvaporscompression cycles could not be combinedsincetheretis `insufhcient feed coming from the second stage of the flash cycle to condensefthe compressedvaporfand turbine exhaust steam Without a prohibitive-rise in vcondensing,temperature and This rise in condensing pressure would raise :cycle y:becomes .nnworkable .The principle f of irecircu- :lation istherneans .by which the "requirementszof 'the vapor compression cycle are matchedfto those of :the

multistage flash cycle.

While the cycle described above employs seven stages of flash evaporation plus vapor compression, a number of variations are possible depending upon specific conditions. For instance the number of stages, the vapor compression ratio, the maximum salt water temperature and the amount of salt water recirculated all may be varied. Choice of a particular cycle of this type in each instance will depend upon an analysis of such factors as the relation of fixed charges for plant and equipment to labor and fuel costs.

The compressor could be operated by other means than steam i. e. by electrical or diesel power. Less recirculation would be required since the turbine exhaust and its latent heat would be eliminated from the cycle. The output for a given compressor power input would be reduced. Overall economy Would probably be less but would in the last analysis depend on an accurate heat balance and evaluation of the relative cost of the different power sources. If diesel power were used, the heat in the diesel engine jacket Water could be added to the evaporator cycle by means of a heat exchanger located in the same point in the cycle as the turbine exhaust salt water heater.

Although the evaporator has been described in connection with the production of fresh water from salt Water, it is adapted to the evaporation of other liquids as well.

We claim:

l. An evaporator comprising a first flash chamber and a second flash chamber, means for compressing vapors produced in the first chamber, a rst heat exchanger wherein said compressed vapors are condensed, a second heat exchanger wherein vapors produced in the second chamber are condensed, a conduit for conducting feed Water successively through said second heat exchanger and said first heat exchanger and into said first flash chamber, a conduit for leading a portion of the feed Water not vaporized in said first flash chamber into said second flash chamber, a conduit for leading the remainder of said unvaporized feed water into said rst heat exchanger.

2. Apparatus for evaporating a liquid comprising a .plurality of :.flashchambers :ein -series from arst toria last .one thereof, ,-al condenser foreach flash chamber except the flrst,..means fortcompressing.vapors produced in `said` firsts-flash chamber, .a condenser for condensing said compressed vapors, means for conducting feed water :through said 4condensers successivelyfromthe `lastffto .theafirst-thence-.throughsaid compressed vapor condenser .andthenceintofsaidflrst flash cha-mbenumeansy for conducting unevaporated feed water from each flash chamber to the'next succeedingflash chamber,.and.means for conicluctinga-.portion offthe u-nevaporated feed.water from the first `flash chamber to the inlet ofsaid last mentioned condenser, and `means for conducting the remainder of the unevaporated feed water from the .first flash chamber successively through the'remainingflashchambers 3. Apparatus for evaporating a liquid, comprising "a flash evaporator having a plurality of. flash chambersfrom a first one to .a last one '.thereof, aflash chamber icondenser associated with each of-said flashchambers except the first, means for conductingfeed waterin out of .contact heatexchange relationship through :said 4condensers, means for rcompressing vapor produced in said first stage, va steam operated prime -mover for said compressing means, means for supplying steam .tovsaidvprime mover, a vapor condenser forcondensing vapors compressed in said .compressing means, av steam condenser for condensing exhauststeam from said prime mover, means for conducting'feed water in out of contact heat-exchange relationship successively through .said flash chambercondenser, said vapor condenser and `said steam .condenser .and thenceinto said first flash chamber, meansfor recirculating aportion of the feed water notevaporatedin .,saidfirst chamber through saidfvapor,condenseraandzsaid .steam condenser, and means for conductingthe residue of ,said .unevaporated' feed i water .successively through the remainder of said flash chambers.

4. Apparatus for evaporating and distilling, comprising a first flash chamber and a second flash chamber, means for maintaining said chambers at successively lower, sub-atmospheric pressures, a condenser for vapors evolved in said second flash chamber, a compressor for vapors evolved in said rst flash chamber, a vapor condenser for condensing vapors compressed in said compressor, a steam-driven prime mover for said compressor,

A a steam condenser for condensing exhaust steam from said prime mover, a conduit for conducting feed water successively through the condenser for said second flash chamber, said compressed vapor condenser and said exhaust steam condenser in out of contact heat exchange relationship with the vapors therein and thence into said first flash chamber, a conduit for conducting a portion of the unvaporized feed water (eflluent) from said flash chamber to the inlet end of said compressed Vapor condenser, and a conduit for conducting the remainder of the unevaporated feed water (eflluent) from said first flash chamber to said second flash chamber.

5. Apparatus for evaporating, comprising a first flash chamber and a second flash chamber, a first condenser for condensing vapors produced in said second flash chamber, a compressor for compressing vapors produced in said rst flash chamber, a second condenser for condensing vapors compressed in said compressor, a steam driven prime mover for said compressor, a third condenser for condensing exhaust steam from said prime mover, means for conducting feed Water successively throughsaid first condenser, said second condenser and said third condenser to heat the same by out of contact heat exchange with the vapors therein, means for conducting said heated feed water into said first flash chamber, means for recirculating a portion of the effluent from said rst flash chamber through said second condenser and said third condenser, means for varying the amount of recirculated eflluent, and means for conducting the remainder of the effluent from said first flash chamber into said second flash chamber.

6. An evaporator including a rst evaporating chamber and a second evaporating chamber, a first Weir contained wihin said first chamber forming an overiiow means for the eluent from said chamber, a second weir contained within said chamber spaced outwardly from said first weir, and forming therewith a container for said effluent, means for withdrawing eiuent from said container and a passage for conducting effluent overowing said second weir to said second evaporating chamber.

7. A flash evaporator including a first ash chamber and a second flash chamber, a container in said first chamber for the distilland, said container including an overflow Weir for the effluent from said container, said Weir being spaced from a wall of said dash chamber, a second overflow weir in said rst flash chamber located between said first Weir and said Wall, means for withdrawing eiuent from the space between said weirs and a passage opening into the wall of said chamber at the lower end thereof for conducting the effluent of the second Weir to the second ash chamber.

8. Method of evaporating comprising vaporizing heated feed water in a rst flash chamber, compressing the vapor evolved in said rst ash chamber, supplying said compressed vapor to a compressed vapor condenser, supplying a portion of the feed water unevaporated in said first flash chamber to said compressed Vapor condenser for out of contact heat exchange with vapors in said compressed vapor condenser, supplying the remainder of said feed water unevaporated in said rst flash chamber to a second flash chamber for flash evaporation therein, supplying the vapor evolved in said second flash chamber to a ash chamber condenser, heating fresh feed water by out of contact heat exchange with vapors in said ash chamber condenser and said compressed vapor condenser successively, and supplying the fresh feed water and the said portion of unevaporated feed water, both of which have been heated in said compressed vapor condenser, to said Erst flash chamber for vaporization therein.

9. Method of evaporating comprising vaporizing heated feed water in a rst ash chamber, compressing the vapor evolved in said rst ash chamber, supplying said compressed vapor to a compressed vapor condenser, supplying a portion of the feed water unevaporated in said first ash chamber to said compressed vapor condenser for out of contact heat exchange with vapors in said compressed vapor condenser, supplying the remainder of said feed water unevaporated in said first flash chamber to a plurality of ilash chambers in succession, supplying the vapor evolved in said flash charnbers to a plurality of associated ilash chamber condensers, heating fresh feed water by out of contact heat exchange with vapors in said Hash chamber condenser and in said compressed vapor condenser successively, and supplying the fresh feed water and the said portion of unevaporated feed water, both of which have been heated in said compressed vapor condenser, to said iirst flash chamber for vaporization therein.

References Cited in the le of this patent UNITED STATES PATENTS 643,702 Waterhouse Feb. 20, 1900 2,185,595 Kleinschmidt Jan. 2, 1940 2,324,663 Aiton July 20, 1943 2,398,068 Worthen et al. Apr. 9, 1946 2,440,173 Henszey Apr. 20, 1948 2,487,884 Lunt Nov. 15, 1949 2,613,177 Worthen et al. Oct. 7, 1952 2,696,465 Kittredge Dec. 7, 1954 

2. APPARATUS FOR EVAPORATING A LIQUID COMPRISING A PLURALITY OF FLASH CHAMBERS IN SERIES FROM A FIRST TO A LAST ONE THEREOF, A CONDENSER FOR EACH FLASH CHAMBER EXCEPT THE FIRST, MEANS FOR COMPRESSING VAPORS PRODUCED IN SAID FIRST FLASH CHAMBER, A CONDENSER FOR CONDENSING SAID COMPRESSED VAPORS, MEANS FOR CONDUCTING FEED WATER THROUGH SAID CONDENSERS SUCCESSIVELY FROM THE LAST TO THE FIRST THENCE THROUGH SAID COMPRESSED VAPOR CONDENSER AND THENCE INTO SAID FIRST FLASH CHAMBER, MEANS FOR CONDUCTING UNEVAPORATED FEED WATER FROM EACH FLASH CHAMBER TO THE NEXT SUCCEEDING FLASH CHAMBER, AND MEANS FOR CON DUCING A PORTION OF THE UNEVAPORATED FEED WATER FROM 